JP3336876B2 - Thin steel sheet with excellent workability and oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet which is excellent in workability and oxidation resistance and can be used in a wide temperature range from room temperature to high temperature, and more particularly to a thin steel sheet suitable for a spiral fin. .

[0002]

2. Description of the Related Art Thin steel sheets finished by hot rolling or cold rolling are used not only for applications at room temperature, such as automobiles, home appliances and building materials, but also for heat exchangers and exhaust gas devices (mufflers, etc.). And is often used for applications subject to high temperature oxidation. It is well known that when a steel sheet is used at a high temperature, surface oxidation proceeds rapidly. In addition, in an environment containing moisture and other factors that promote oxidation even at normal temperature, oxidative rust occurs during long-term use, causing a major problem of a reduction in the wall thickness of the member. As a measure for preventing oxidation of the steel sheet, there are surface treatments such as plating and painting, but it can be said that it is most desirable that the steel sheet itself has oxidation resistance. As a means for imparting oxidation resistance to a steel sheet, the addition of Cr into steel is generally used.
It is known that the maximum corrosion depth is improved when the amount of Cr added is not less than wt%.

[0003] On the other hand, however, it is well known that the addition of Cr deteriorates the workability of the steel, particularly the cold workability, and easily causes brittle cracking of the steel. Examples of the brittle cracks include slab cracks in the manufacturing process of thin steel sheets, edge cracks during cold rolling, weld cracks, etc., and cracks in which ductility and brittleness are mixed during use after shipping, for example, deep drawing. Cracks that occur when trying to apply further processing afterwards, cracks that occur when stress is repeatedly applied to the welded part after processing, cracks that occur when heating and cooling are repeated on the processed one,
This is an edge crack or the like that occurs when the slit is processed and then further processed.

[0004] When Cr is added to steel as described above, C
As the amount of r added increases, the weight loss due to oxidation decreases, and the oxidation resistance improves, but the workability and brittle crack resistance decrease. Therefore, when Cr is added to a thin steel sheet requiring excellent workability to improve oxidation resistance, careful consideration must be taken to suppress the deterioration of the workability and the brittle crack resistance as described above. is necessary. In order to meet such demands, Japanese Patent Publication No. 61-35881 discloses a low carbon Cr.
There has been proposed an invention relating to a steel sheet having improved high-temperature oxidation resistance without impairing workability by adding a small amount of Ti to steel. However, since the present invention is aimed exclusively at steel plates for spiral fins, no consideration is given to brittle crack resistance, and sufficient characteristics cannot be obtained in this regard.

On the other hand, Japanese Patent Publication No. 5-75820 discloses a Cr-containing thin steel sheet having particularly improved workability and corrosion resistance with improved brittle crack resistance, C: 0.020 wt% or less, and Si: 1. 0 wt% or less, Mn: 0.10 to 2.0
wt%, S: 0.009 wt% or less, Cr: 0.8-
6.0 wt%, acid-soluble Al: 0.005 to 0.30 wt
%, N: 0.0060 wt% or less, Ti: 0.01 to
0.30 wt%, B: 0.0002 to 0.0012 wt
%, The balance being Fe and unavoidable impurities, and T
i ≧ 4 [C + 12/14) N + (12/32) S-0.
0010], B / Cr ≧ 0.0001, Mn / S ≧ 20
We propose a thin steel sheet with excellent workability and oxidation resistance.

[0006]

[Problems to be solved by the invention]
For example, the steel sheet according to No.-75820 is remarkably oxidized by heat when it is welded after winding around a spiral fin, and has a problem in oxidation resistance. Further, in terms of workability, there is also a problem that cracks are generated during winding processing or welding depending on the shape of the spiral fin to be processed (when the size is large or the degree of processing is large). Furthermore, when slitting a steel sheet to a narrow width, burrs are greatly generated depending on the processing conditions, which causes a problem in manufacturing spiral fins. Accordingly, it is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a thin steel sheet having both excellent workability and oxidation resistance.

[0007]

Means for Solving the Problems The inventors of the present invention have found from the results of many tests that the content of specific component elements and the cleanliness in steel should be regulated in steel sheets to which Cr is added for improving oxidation resistance. Has been found to provide excellent workability (suppression of cracking during winding, suppression of cracking during welding, suppression of burrs during slitting) and oxidation resistance (particularly suppression of surface oxidation due to heat during welding). The gist of the present invention lies in the following points (1) to (3) in a Cr-containing thin steel sheet containing 0.8 to 1.2 wt% of Cr.

(1) The oxidation resistance is drastically improved by keeping the amounts of Mn and Si below a predetermined level and adding a small amount of Cu. (2) In order to obtain excellent workability in the component system in which the Mn content and the Si content are reduced, each content of C, S, P, O, and N is set to a predetermined level or less, and then Ti is appropriately added. Added.
Further, as means for improving workability, control of the amount of O by adding an appropriate amount of Al and control of cleanliness in steel within the specified amount of O are performed. (3) In order to suppress burrs at the time of slitting, the amount of Mn is set to a predetermined level or less, and a small amount of B is added.

That is, the feature of the present invention is that C: 0.005
wt% or less, Si: 0.05 wt% or less, Mn: 0.1
5 wt% or less, S: 0.005 wt% or less, P: 0.0
15 wt% or less, O: 0 to 0.0040 wt%, Cr:
0.8 to 1.2 wt%, Al: 0.005 to 0.100
wt%, N: 0 to 0.0050 wt%, Ti: 0.02
0 to 0.300 wt%, B: 0.0001 to 0.000
5 wt%, Cu: 0.01 to 0.05 wt%,
Further, it is a thin steel sheet excellent in workability and oxidation resistance, having a cleanliness (dt) specified by JIS G 0555 of 0.05% or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention and the reasons for limitation will be described below. The steel sheet targeted in the present invention is 600 ° C.
It is a thin steel sheet with small oxidation loss at a high temperature up to the extent, and therefore it is fundamental to contain Cr. Where Cr
If the amount is less than 0.8 wt%, the desired effect of adding Cr cannot be obtained, while if it exceeds 1.2 wt%, workability as intended by the present invention cannot be obtained. For this reason, the amount of Cr is 0.
8 to 1.2 wt%. 0.8-1.2w like this
In order to sufficiently improve the oxidation resistance of the t% Cr-containing steel sheet, it is essential to reduce the amounts of Mn and Si and to add a small amount of Cu.

Mn causes remarkable surface oxidation due to heat during welding, and the workability intended by the present invention (suppression of edge cracking during winding, suppression of burr during slitting).
Is also a harmful element that deteriorates, and therefore it is necessary to reduce it sufficiently. If the Mn content exceeds 0.15 wt%, the surface oxidation becomes remarkable, and the processability also deteriorates.
The Mn content is 0.15 wt% or less. Si is also a harmful element that causes significant surface oxidation due to heat during welding.
If the i content exceeds 0.05 wt%, the surface oxidation becomes remarkable, so the Si content is set to 0.05 wt% or less. Cu is an essential additive element for improving the oxidation resistance intended by the present invention. If the Cu content is less than 0.01 wt%, the desired effect cannot be obtained. On the other hand, if the Cu content exceeds 0.05 wt%, the workability (suppression of cracking during welding) deteriorates.
01-0.05 wt%.

In order to obtain excellent workability under the condition that the Mn content is 0.15 wt% or less, which is lower than that of the conventional steel sheet, as in the present invention, each of C, S, P, O and N is required. Control of the content is important. C impairs the workability of the Cr-added steel sheet targeted by the present invention (particularly, suppression of cracking during spiral fin winding processing), and when the amount of C is large, the amount of Ti added to fix C as carbides. And the cost increases. Therefore, the C content is set to 0.005 wt% or less. The C content is preferably as small as possible in the range of 0.005 wt% or less.

S forms a sulfide such as MnS or TiS. When these sulfides are finely precipitated in the steel, they do not particularly impair the properties of the steel sheet.
When the content exceeds t%, sulfides are coarsely precipitated, and the workability of the Cr-added steel sheet targeted by the present invention (particularly, crack suppression during spiral fin processing and crack suppression during welding) is deteriorated. Therefore, the amount of S is set to 0.005 wt% or less. The amount of S is 0.
It is preferable that the amount is as small as possible in the range of 005 wt% or less. P needs to be sufficiently reduced in order to improve the workability (particularly, suppression of cracking during welding) of the Cr-added steel sheet targeted by the present invention. P content is 0.015wt%
If it exceeds 0.5%, cracks occur during the winding process on the spiral fin. The P content is preferably as small as possible, and particularly preferably 0.007 wt% or less is a more preferable level.

O is a harmful element that deteriorates the workability of the Cr-added steel sheet targeted by the present invention. O content is 0.0040
If the content is more than wt%, cracks will occur during the winding process on the spiral fin. Therefore, the content is set to 0.0040 wt% or less (including the case of no addition). The smaller the amount of O, the better,
In particular, 0.0025 wt% or less is a more preferable level. Al is an element added for deoxidation adjustment in the steelmaking stage, but if it is less than 0.005 wt%, deoxidation becomes insufficient,
On the other hand, if it exceeds 0.100 wt%, fine alumina particles will remain in the steel, so that the Al content is 0.005 wt%.
To 0.100 wt%.

Since N impairs the workability of the Cr-added steel sheet targeted by the present invention (the suppression of cracking during spiral fin winding), it is preferable to reduce N as much as possible. In addition, N consumes Ti as TiN as described later. Therefore, if the amount of N is large, the amount of added Ti must be increased, which is disadvantageous in terms of cost. Therefore, the amount of N is set to 0.0050 wt% or less (including the case of no addition). Ti is an important element in the present invention, and fixes C, N, and S in the steel to form precipitates such as TiC, TiN, and TiS, thereby cleaning the steel, thereby improving the workability intended by the present invention. Has the effect of improving. If the amount of Ti is less than 0.020 wt%, the desired effect cannot be obtained by the above operation, and the effect of improving workability is small. On the other hand, if the amount of Ti exceeds 0.300 wt%, solid solution Ti increases and the effect of improving workability is saturated,
On the contrary, the cost increases. Therefore, the amount of Ti is 0.020 to
0.300 wt%.

B is an indispensable additive element for suppressing burrs during slit processing in the present invention. FIG. 1 shows a steel sheet in which the contents and cleanliness (dt) of C, Si, S, P, O, Cr, Al, N, Ti, and Cu satisfy the conditions of the present invention, and the Mn content is 0.15 wt. 2 shows the results of examining the effect of the addition of B on the burr height and workability for steel sheets having a percentage of not more than (%) and more than 0.15 wt% (•). According to the figure, in a steel sheet having an Mn content of 0.15 wt% or less, the burr height is reduced to a preferable level of 5 μm or less when the B content is 0.0001 wt% or more. On the other hand, if the B content exceeds 0.0005 wt%, workability (suppression of cracking during winding processing on the spiral fin) is deteriorated. Further, in the steel sheet having an Mn content of more than 0.15 wt%, the workability is inferior regardless of the amount of B added, and the burr height is also more than 5 μm. Inferior. From the above results, in the present invention, the amount of B added is set to 0.0001 to 0.0005 wt as a range in which both burr suppression and workability during slit processing can be compatible.
%.

Although the steel sheet of the present invention is substantially composed of Fe except for the above-mentioned respective component elements, it does not prevent the inclusion of other elements as long as the above-mentioned effects of the present invention are not impaired. In the present invention, the regulation of cleanliness (dt) specified by JIS G 0555 is an important requirement for improving the workability of the Cr-added steel sheet targeted by the present invention. Even if the O content and the S content of the steel sheet are within the above-mentioned range of the present invention, JIS G
If the cleanliness (dt) specified by 0555 exceeds 0.05%, the workability intended by the present invention (suppression of cracking during winding on the spiral fin) cannot be obtained, and thus the cleanliness (dt) is 0. 0.05% or less. In addition, this cleanliness:
The content of 0.05% or less can be achieved by appropriately performing out-of-furnace refining as well as restricting the amount of Al and the amount of O.

Although there is no particular limitation on the method for producing the thin steel sheet of the present invention, it can be usually produced by the following production method. That is, a steel having the above-described composition is formed into a slab by continuous casting, and the slab is hot-rolled and used as a hot-rolled steel sheet, or further cold-rolled and used as a cold-rolled steel sheet. The continuous casting and hot rolling may be performed continuously (that is, by direct-feed rolling). There are no special restrictions on the conditions of the continuous casting, hot rolling and cold rolling, and they can be performed under ordinary conditions. The hot-rolled steel sheet may be annealed as it is during hot rolling or after pickling. In addition, before the pickling, a pretreatment is performed by a mechanical method or the like to improve the scale removal property by the pickling.

When cold rolling is performed after pickling,
Annealing is performed after cold rolling. This annealing can be carried out by either box annealing or continuous annealing, but continuous annealing that allows short-time annealing is more preferable. The annealing temperature is desirably about 650 to 900 ° C. in the case of continuous annealing. Annealing temperature is 650 ℃
If the temperature is less than 900 ° C., the workability is insufficient. It is also effective that the hot-rolled steel sheet and the cold-rolled steel sheet thus obtained are further subjected to surface treatment such as plating and painting to improve corrosion resistance and oxidation resistance. When performing hot-dip plating, annealing and plating can also be performed in a hot-dip plating line.

[0020]

EXAMPLES Steel having the components shown in Table 1 was melted in a converter, refined outside the furnace,
After passing through the steps of continuous casting, hot rolling, pickling, and cold rolling, a cold-rolled steel sheet having a sheet thickness of 1.25 mm x a sheet width of 1000 mm was subjected to continuous annealing or box annealing under the annealing conditions shown in Table 2. . In addition, No. For the test material of No. 1, after continuous casting,
Subsequently, hot rolling (so-called direct rolling) was continuously performed. The cleanliness of the obtained cold-rolled steel sheet was measured based on JIS G 0555 (see Table 2). 14mm of the above cold rolled steel sheet
The slit was made to have a width, and the height of a burr generated at the time of slitting was examined. Further, the slit coil was wound around a pipe having an outer diameter of 32 mm, and the occurrence of edge cracking at that time was examined. Further, after the winding, welding was performed, and the state of surface oxidation thereafter was visually observed, and the presence or absence of cracking due to welding was examined with a stereoscopic microscope. Table 3 shows the results.

According to Table 3, No. 1 of the present invention was used. 1 to
No. 4 is workability (control of edge cracking during winding processing,
It is excellent in suppressing cracks during welding and suppressing burrs during slitting, and has low oxidation on the surface during welding, indicating good oxidation resistance. On the other hand, in the case of No. 3 in which the amount of Si exceeded the upper limit of the present invention. In Comparative Example No. 5, the surface oxidation during welding was remarkable. Further, in the case of No. 3 in which the amount of Mn exceeded the upper limit of the present invention. In Comparative Example No. 6, edge cracking occurred during winding, surface oxidation during welding was remarkable, and burr height during slitting exceeded 5 μm, all of which were inferior to those of the present invention. I have. In addition, in the case of No. 3 in which the amount of S exceeded the upper limit of the present invention. In the comparative example of No. 7, edge cracking occurred during winding and cracking occurred during welding. Also,
No. P in which the amount of P exceeded the upper limit of the present invention. In the comparative example of No. 8, the surface roughness of the edge during the winding process occurs, and the crack during the welding also occurs.

Further, when the amount of O exceeds the upper limit of the present invention, N
o. In Comparative Example No. 9, the cleanliness of the steel sheet also exceeded the upper limit of the present invention, and edge cracking occurred during winding processing. According to this comparative example, it is clear that an increase in the amount of O lowers the cleanliness and causes deterioration in workability. In addition, No. 3 in which the amount of Cu exceeded the upper limit of the present invention. In the comparative example of No. 10, cracks occurred during welding. In addition, No. B in which the amount of B exceeded the upper limit of the present invention. In the comparative example of No. 11, edge cracking occurred during winding processing. On the other hand, No. B in which the B amount is less than the lower limit of the present invention. Twelve comparative examples have a burr height of more than 5 μm during slit processing, and are inferior to those of the present invention.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

As described above, the thin steel sheet of the present invention has excellent oxidation resistance and workability. In particular, spiral fins have excellent processing characteristics, are free from edge cracking during winding processing, have almost no discoloration due to surface oxidation during welding, have no cracking during welding, and have low burr height during slit processing, and are highly manufacturable. Is excellent. Therefore, it can be used as a material steel plate for various members, such as automobiles, home electric appliances, heat exchangers, building materials, and the like, for which a variety of applications requiring workability and oxidation resistance at room temperature or higher are required. Also, having excellent oxidation resistance means that oxidation loss is small even when used at normal temperature as well as at high temperature. The effect is great.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of B added, workability, and burr height during slit processing.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroshi Sawada 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-5-209228 (JP, A) JP-A-5 JP-195078 (JP, A) JP-A-4-22125 (JP, A) JP-A-3-199342 (JP, A) JP-A-63-230854 (JP, A) JP-A-63-230853 (JP, A) (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] C: 0.005 wt% or less, Si: 0.1% or less.
05 wt% or less, Mn: 0.15 wt% or less, S: 0.
005 wt% or less, P: 0.015 wt% or less, O: 0
0.0040 wt%, Cr: 0.8-1.2 wt%,
Al: 0.005 to 0.100 wt%, N: 0 to 0.0
050 wt%, Ti: 0.020 to 0.300 wt%,
B: 0.0001 to 0.0005 wt%, Cu: 0.0
1 to 0.05 wt%, and JIS G 0555
A thin steel sheet having excellent workability and oxidation resistance having a cleanliness (dt) of 0.05% or less as specified in (1).